Fabrics, rugs, upholstery, and similar articles are often cleaned with devices called "extractors" which wet the item to be cleaned with a cleaning liquid which is then sucked up by vacuum. Such extractors typically include the following elements: a tool acting as a cleaning head (wand) that is systematically moved over the fabric to clean it; a holding tank for containing cleaning liquid (plain water, or other cleaning liquids such as an aqueous solution of detergent, surfactant, etc.); a pump; a delivery line from the tank, through the pump, and to the tool head for spraying cleaning liquid onto the fabric; a vacuum opening on the wand for sucking up cleaning liquid from the object being cleaned; and a vacuum source (such as vacuum cleaner) attached to the cleaning head through a flexible conduit or hose.
Extractor spray head nozzles and pumps should be adapted to spray cleaning liquid with the force appropriate to the item being cleaned. Rugs require a deep penetration and high-velocity sprays, while upholstery should have a low-velocity spray so that underlying layers of foam rubber and backing are not over-wetted. If these underlying layers are over-wetted the cleaning liquid will pick up their dirt and that dirt will be carried up to the fabric surface by the vacuum; the dirt will continue to bleed outward and the outer fabric then cannot be cleaned. Also, the cleaned item will require a long time to dry.
Interchangeable cleaning heads may be provided for various different items which require different penetration depths. The different heads may include different nozzle patterns or total nozzle orifice areas.
Because of the different levels of spray force needed for various items, extractors have been designed to provide adjustable spray force levels. Extractors are divided generally into to classes: high-pressure and low-pressure.
The low-pressure type (delivery line pressures up to about 100 psi) typically uses an on-demand diaphragm pump. The pump will run whenever a trigger switch is thrown by the user, or, when a pressure sensor senses a drop caused by a trigger spray valve. An on-demand pump system requires no bypass valve from the delivery line back to the holding tank; all the cleaning liquid sucked from the holding tank goes directly out of the spray head. A diaphragm pump is not a positive-displacement type of pump.
A high-pressure extractor (over 100 psi delivery line pressure) typically uses a positive-displacement pump, such as a piston pump. Due to their heavier construction and the added inertia of the flywheel, these pumps are not easy to start and stop and a simple switch of the power from abruptly on to off is not satisfactory. When the user triggers the spray head on and off, the pressure in the delivery line must somehow be compensated, and pressure spikes caused by liquid surges allowed for. Pressure in the cleaning fluid delivery line can become too high from surge, or during steady spraying if there is any slight variation in the liquid or minor clogging due to dirt.
High-pressure extractors all use a bypass valve in the delivery line, downstream of the pump, to lower the pressure by bleeding cleaning fluid back into the holding tank when the trigger spray valve is closed; otherwise the flow would be blocked and the delivery line pressure would become excessive. To provide different spray head pressures, the prior-art extractors use two different systems. Either the bypass valve is adjusted, or, the motor speed is adjusted.
The high-pressure pumps are driven by an electric motor of either the AC or the DC type.
AC motors cannot be run at different speeds because they turn with the constant 60-Hz cycles of line voltage (unless they use a very sophisticated motor speed control). A positive-displacement pump delivers a positive amount of liquid, and this liquid must always be going somewhere when the pump is running or the pump will lock up. A non-positive displacement type of pump does not deliver a positive amount of liquid, by design; therefore, the more hydraulic resistance this type of pump sees, the lower the actual pressure and flow that will come out of the tip of the cleaning tool spray head.
Thus, it is possible to use an AC motor in a high-pressure extractor with a positive-displacement pump, but the spray head pressure can only be varied by adjusting the bypass valve relief pressure setting. In order to vary the pressure by varying the motor speed, a special AC power supply that rectifies the line current and then generates AC at a desired frequency must be used; however such units are complex and very expensive, adding up to $300 to the manufacturing cost.
A DC motor will run efficiently at any desired speed and therefore can efficiently pump liquid at any rate when driving a positive-displacement pump, but it must be supplied with a variable voltage. Variable-voltage sources for DC motors are well-known and practical, but they are expensive because of the high-power capacity variable-voltage power supply that is needed. (A DC power supply is roughly the first half of a variable-frequency AC supply, so it costs less but it is still quite costly.) A typical DC power supply includes a control potentiometer that control the cut-off voltage of a bank of SCR's (silicon controlled rectifiers) which "chop" line AC before it is rectified, to control the average voltage fed to the motor. The control circuitry, high-wattage SCR's, and heavy-duty rectifier are not only costly but also consume energy that is thrown off as heat, and of course are liable to malfunction. In addition, a DC motor alone is more expensive than an AC motor of equivalent power.
Because of the expense of varying the motor speed to control the delivery line pressure, another method is often used. An AC motor runs at constant speed and delivers a constant-volume flow, and excess volume is diverted back to the holding tank through a spring-loaded bypass valve. The bypass valve is typically spring-loaded (pressure-activated) piston valve. The spring-loaded piston slides to-and-fro in a cylinder; when pressure is low, the piston is urged forward by the spring to a position in which it blocks return flow to the holding tank; when pressure rises, the piston forces the spring back, uncovering the return-flow orifice. Thus, as the user trigger the spray head and the delivery line pressure jumps up and down, the bypass valve compensates and maintains an even pressure while the motor turns at a constant speed.
The drawback of manually adjusting a bypass valve is that is must be easily accessible. If located inside the housing, the housing must be opened each time that the valve is adjusted; this puts the operator in a potentially hazardous position, since the valve is usually adjusting with the machine on. If located so that the adjusting portion is outside the housing, the valve is liable to freezing and damage during use or transport. Manual adjustment of a conventional bypass valve requires the use of tools. Moreover, continual adjustment of the bypass valve will reduce its life and performance.
There is also the concern that the operator will exceed the factory setting, which would prematurely wear out the pump and motor; depending on how far past the factory setting the adjustment were pushed, the gauge hoses and heat exchanger could also be damaged. Conversely, if the pressure is adjusted too low the adjustment nut may actually be unscrewed causing the cleaning liquid to go everywhere and the piston and spring to be lost. Loss of the nut can also occur due to the operator's failure to properly lock it in place with the lock nut that is provided to prevent loosening due to vibration. Tightening of the lock nut also requires the use of tools in most cases.
The problems of adjusting a manual bypass valve limit the usefulness of prior-art high-pressure extractors. Deep-penetration, high-velocity spray items like carpets and shallow-penetration, low-velocity spray items like upholstery cannot be alternately cleaned with the same extraction machine. The user cannot readily adjust the delivery line pressure between the higher pressure needed for carpets and the lower pressure needed for upholstery.
The user might also want to quickly adjust the pressure when mounting various interchangeable spray heads on the wand, or for other reasons.
The prior art does not disclose any simple, foolproof, and inexpensive apparatus for easily and quickly switching cleaning fluid delivery pressures in an extraction-type cleaner.